Seatguru.com has info on the Qantas seat pitch. However it only covers Jetstar's A330, not the A320 used domestically.
From experience the jetstar pitch is close to the Qantas B737 pitch.
Qantas has basic in flight catering included, Jetstar is a LCC. Buy something if you wish.
The sectors are around 1 hour so from a comfort point of view you won't suffer very long.
Jetstar have check in on the internet, or at check in counter at the airport.
Also self service machines in the check in area at the airport. Same for Qantas.
All in all, it comes down to how important the 40 bucks difference is to you. The difference in the travel experience is not great especially over a short sector.
Enjoy your visit.